URLUtils.search returns empty string, not '?'

From the latest draft http://url.spec.whatwg.org/#dom-url-search:

>The search attribute must run these steps:
>
>If url is null, or its query is either null or the empty string, return the empty string.
>
>Return "?" concatenated with query.

And from 2012 https://dvcs.w3.org/hg/url/raw-file/tip/Overview.html#dom-url-search

>The search attribute must run these steps:
>
>If the fatal error flag is set, or query is either null or the empty string, return the empty string.
>
>Return "?" concatenated with query.

In each of these cases, the expected result of `location.search` for url `./support/WorkerLocation.js?` is the empty string.
Merge pull request #984 from caitp/patch-3

URLUtils.search returns empty string, not '?'